Acoustic wave device with spinel layer and temperature compensation layer

ABSTRACT

An acoustic wave device that includes a spinel layer, a piezoelectric layer, a temperature compensating layer between the spinel layer and the piezoelectric layer and an interdigital transducer electrode on the piezoelectric layer is disclosed. The piezoelectric layer is disposed between the interdigital transducer electrode and the spinel layer. The acoustic wave device is configured to generate an acoustic wave having a wavelength of λ. The piezoelectric layer can have a thickness that is less than λ. In some embodiments, the spinel layer can be a polycrystalline spinel layer.

CROSS REFERENCE TO PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR § 1.57.This application is a continuation of U.S. patent application Ser. No.16/430,078, filed Jun. 3, 2019, and titled “ACOUSTIC WAVE DEVICE WITHSPINEL LAYER AND TEMPERATURE COMPENSATION LAYER,” which claims thebenefit of priority of U.S. Provisional Patent Application No.62/683,333, filed Jun. 11, 2018 and titled “ACOUSTIC WAVE DEVICE WITHSPINEL LAYER,” and also claims the benefit of priority of U.S.Provisional Patent Application No. 62/738,245, filed Sep. 28, 2018 andtitled “ACOUSTIC WAVE DEVICE WITH SPINEL LAYER,” the disclosures of eachof which are hereby incorporated by reference in their entiretiesherein.

BACKGROUND Technical Field

Embodiments of this disclosure relate to acoustic wave devices.

Description of Related Technology

Acoustic wave filters can be implemented in radio frequency electronicsystems. For instance, filters in a radio frequency front end of amobile phone can include acoustic wave filters. Two acoustic wavefilters can be arranged as a duplexer.

An acoustic wave filter can include a plurality of resonators arrangedto filter a radio frequency signal. Example acoustic wave filtersinclude surface acoustic wave (SAW) filters and bulk acoustic wave (BAW)filters. A surface acoustic wave resonator can include an interdigitaltransductor electrode on a piezoelectric substrate. The surface acousticwave resonator can generate a surface acoustic wave on a surface of thepiezoelectric layer on which the interdigital transductor electrode isdisposed.

SUMMARY

The innovations described in the claims each have several aspects, nosingle one of which is solely responsible for its desirable attributes.Without limiting the scope of the claims, some prominent features ofthis disclosure will now be briefly described.

In one aspect, an acoustic wave device is disclosed. The acoustic wavedevice includes a polycrystalline spinel layer and a piezoelectriclayer. The piezoelectric layer has a thickness than is less than λ. Theacoustic wave device is configured to generate an acoustic wave having awavelength of λ. The acoustic wave device also includes an interdigitaltransducer electrode on the piezoelectric layer. The piezoelectric layeris disposed between the interdigital transducer electrode and thepolycrystalline spinel layer.

The acoustic wave device can further include a temperature compensatinglayer disposed between the polycrystalline spinel layer and thepiezoelectric layer. The temperature compensating layer can includesilicon dioxide. The acoustic wave device can further include a highimpedance layer disposed between the temperature compensating layer andthe polycrystalline spinel layer, in which the high impedance layer hasa higher acoustic impedance than the polycrystalline spinel layer. Thetemperature compensating layer can be in physical contact with thepiezoelectric layer. The temperature compensating layer can be inphysical contact with the polycrystalline spinel layer that is oppositeto the piezoelectric layer.

The polycrystalline spinel layer can be in physical contact with thepiezoelectric layer.

The piezoelectric layer can be a lithium tantalate layer or a lithiumniobate layer.

The acoustic wave device can further includes a substrate layer. Thepolycrystalline spinel layer can be disposed between the substrate layerand the piezoelectric layer. The substrate layer can include one of asilicon layer, an aluminum nitride layer, a diamond layer, a siliconnitride layer, or a silicon carbide layer. The substrate layer can be inphysical contact with the polycrystalline spinel layer. The acousticwave device can further include an adhesive layer attaching thesubstrate layer and the polycrystalline spinel layer.

In one aspect, a radio frequency module is disclosed. The radiofrequency module includes an acoustic wave filter that is configured tofilter a radio frequency signal. The acoustic wave filter includes anacoustic wave device that is configured to generate an acoustic wavehaving a wave length of λ. The acoustic wave device includes apiezoelectric layer having a thickness of less than λ, an interdigitaltransducer electrode on the piezoelectric layer, and a polycrystallinespinel layer that is arranged such that the piezoelectric layer isdisposed between the polycrystalline spinel layer and the interdigitaltransducer electrode. The radio frequency module also includes a packageenclosing the acoustic wave filter.

The radio frequency module can further include a duplexer, in which theduplexer includes the acoustic wave filter and a second acoustic wavefilter.

The radio frequency module can further include a radio frequency switchcoupled to the acoustic wave filter. The radio frequency switch can beenclosed within the package. The radio frequency module can furtherinclude a power amplifier enclosed within the package.

The acoustic wave device can further include a temperature compensatinglayer that is disposed between the polycrystalline spinel layer and thepiezoelectric layer. The acoustic wave can also include a high impedancelayer that is disposed between the temperature compensating layer andthe polycrystalline spinel layer, in which the high impedance layer hasa higher acoustic impedance than the polycrystalline spinel layer.

The acoustic wave device can further include a substrate layer. Thepolycrystalline spinel layer can be disposed between the substrate layerand the piezoelectric layer.

In one aspect, a wireless communication device is disclosed. Thewireless communication device includes an antenna and an acoustic wavefilter that is in communication with the antenna. The acoustic wavefilter is configured to filter a radio frequency signal. The acousticwave filter includes an acoustic wave device that is configured togenerate an acoustic wave having a wave length of λ. The acoustic wavedevice includes a piezoelectric layer having a thickness of less than λ,an interdigital transducer electrode on the piezoelectric layer, and apolycrystalline spinel layer arranged such that the piezoelectric layeris disposed between the polycrystalline spinel layer and theinterdigital transducer electrode.

The acoustic wave device can further include a temperature compensatinglayer that is disposed between the polycrystalline spinel layer and thepiezoelectric layer. The acoustic wave device can also include a highimpedance layer that is disposed between the temperature compensatinglayer and the polycrystalline spinel layer, in which the high impedancelayer has a higher acoustic impedance than the polycrystalline spinellayer.

In one aspect, an acoustic wave device is disclosed. The acoustic wavedevice includes a piezoelectric layer, a spinel layer and a temperaturecompensating layer that is disposed between the piezoelectric layer andthe spinel layer. The acoustic wave device also includes an interdigitaltransducer electrode on the piezoelectric layer. The piezoelectric layeris disposed between the temperature compensating layer and theinterdigital transducer electrode.

The spinel layer can be a polycrystalline spinel layer. Alternatively,the spinel layer can be a single crystalline spinel layer.

The piezoelectric layer can be a lithium tantalate layer or a lithiumniobate layer.

The acoustic wave device is configured to generate an acoustic wavehaving a wavelength of λ, and the piezoelectric layer can have athickness of less than λ.

The temperature compensating layer can include silicon dioxide. Thetemperature compensating layer has a first surface and a second surface.The first surface can be in physical contact with the spinel layer. Thesecond surface can be in physical contact with the piezoelectric layer.

The acoustic wave device can further include a substrate layer. Thespinel layer can be disposed between the substrate layer and thepiezoelectric layer. The substrate layer can include one of a siliconlayer, an aluminum nitride layer, a diamond layer, a silicon nitridelayer, or a silicon carbide layer.

The acoustic wave device can further include a high impedance layerdisposed between the temperature compensating layer and the spinellayer, in which the high impedance layer has a higher acoustic impedancethan the polycrystalline spinel layer

In one aspect, a radio frequency module is disclosed. The radiofrequency module includes an acoustic wave filter that is configured tofilter a radio frequency signal. The acoustic wave filter includes anacoustic wave device that includes a piezoelectric layer, a spinellayer, and a temperature compensating layer that is disposed between thepiezoelectric layer and the spinel layer. The radio frequency modulealso includes a package enclosing the acoustic wave filter.

The radio frequency module can further include a duplexer. The duplexercan include the acoustic wave filter and a second acoustic wave filter.

The radio frequency module further can include a radio frequency switchcoupled to the acoustic wave filter. The radio frequency switch can beenclosed within the package. The radio frequency module can furtherinclude a power amplifier enclosed within the package.

The spinel layer can be a polycrystalline spinel layer. Alternatively,the spinel layer can be a single crystalline spinel layer.

The piezoelectric layer can be a lithium tantalate layer or a lithiumniobate layer.

The acoustic wave device is configured to generate an acoustic wavehaving a wavelength of λ. The piezoelectric layer can have a thicknessof less than λ.

In one aspect, a wireless communication device is disclosed. Thewireless communication device includes an antenna and an acoustic wavefilter that is in communication with the antenna. The acoustic wavefilter is configured to filter a radio frequency signal. The acousticwave filter includes an acoustic wave device that includes apiezoelectric layer, a spinel layer, and a temperature compensatinglayer that is disposed between the piezoelectric layer and the spinellayer.

The wireless communication device can include a duplexer. The duplexercan include the acoustic wave filter and a second acoustic wave filter.

The wireless communication device can further include a radio frequencyswitch that is coupled to the acoustic wave filter. The radio frequencyswitch and a power amplifier can be enclosed within the package.

The spinel layer can be a polycrystalline spinel layer. Alternatively,the spinel layer can be a single crystalline spinel layer.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the innovations have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment. Thus, theinnovations may be embodied or carried out in a manner that achieves oroptimizes one advantage or group of advantages as taught herein withoutnecessarily achieving other advantages as may be taught or suggestedherein.

The present disclosure relates to U.S. patent application Ser. No.16/429,900, titled “ACOUSTIC WAVE DEVICE WITH SPINEL LAYER,” filed onJun. 3, 2019, the entire disclosure of which is hereby incorporated byreference herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of this disclosure will now be described, by way ofnon-limiting example, with reference to the accompanying drawings.

FIG. 1A illustrates a cross section of a baseline surface acoustic wavedevice.

FIG. 1B illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate according to anembodiment.

FIG. 2A is a graph comparing simulated frequency responses of thesurface acoustic wave devices of FIGS. 1A and 1B.

FIG. 2B is another graph comparing simulated frequency responses of thesurface acoustic wave devices of FIGS. 1A and 1B.

FIG. 2C is a graph comparing admittance over frequency for a surfaceacoustic wave device of FIG. 1B and a similar device without a spinelsubstrate.

FIG. 3A illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate and a temperaturecompensating layer according to an embodiment.

FIG. 3B illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate, a temperaturecompensating layer, and a high impedance layer according to anembodiment.

FIG. 4A illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate over another substrateaccording to an embodiment.

FIG. 4B illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate and a temperaturecompensating layer over another substrate according to an embodiment.

FIG. 4C illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate, a temperaturecompensating layer, and a high impedance layer over another substrateaccording to an embodiment.

FIG. 5A illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate attached to anothersubstrate by an adhesive layer according to an embodiment.

FIG. 5B illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate attached to anothersubstrate by an adhesive layer according to another embodiment.

FIG. 5C illustrates a cross section of a surface acoustic wave devicethat includes a polycrystalline spinel substrate attached to anothersubstrate by an adhesive layer according to another embodiment.

FIG. 6A illustrates a cross section of a baseline surface acoustic wavedevice.

FIG. 6B illustrates a cross section of a surface acoustic wave devicethat includes a spinel substrate according to an embodiment.

FIG. 6C illustrates a cross section of a surface acoustic wave devicethat includes a spinel substrate according to another embodiment.

FIG. 7A is a graph comparing simulated frequency responses of thesurface acoustic wave devices of FIGS. 6A and 6B for a firstpiezoelectric layer thickness.

FIG. 7B is a graph comparing simulated frequency responses of thesurface acoustic wave devices of FIGS. 6A and 6B for a secondpiezoelectric layer thickness.

FIG. 7C is a graph comparing simulated frequency responses of thesurface acoustic wave devices of FIGS. 6A and 6B for a thirdpiezoelectric layer thickness.

FIG. 7D is a graph of electromechanical coupling coefficient (k²) versuslithium tantalate thickness for various silicon dioxide layerthicknesses for the surface acoustic wave devices shown of FIG. 6B.

FIG. 8A is a schematic block diagram of a module with a filter thatincludes a surface acoustic wave device according to an embodiment.

FIG. 8B is a schematic block diagram of a module with a filter thatincludes a surface acoustic wave device according to another embodiment.

FIG. 8C is a schematic block diagram of a module with a filter thatincludes a surface acoustic wave device according to another embodiment.

FIG. 8D is a schematic block diagram of a module with duplexers thatinclude a surface acoustic wave device according to an embodiment.

FIG. 8E is a schematic block diagram of a module with duplexers thatinclude a surface acoustic wave device according to another embodiment.

FIG. 8F is a schematic block diagram of a module with duplexers thatinclude a surface acoustic wave device according to another embodiment.

FIG. 9A is a schematic block diagram of a wireless communication devicethat includes a filter in accordance with one or more embodiments.

FIG. 9B is a schematic block diagram of another wireless communicationdevice that includes a filter in accordance with one or moreembodiments.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following description of certain embodiments presents variousdescriptions of specific embodiments. However, the innovations describedherein can be embodied in a multitude of different ways, for example, asdefined and covered by the claims. In this description, reference ismade to the drawings where like reference numerals can indicateidentical or functionally similar elements. It will be understood thatelements illustrated in the figures are not necessarily drawn to scale.Moreover, it will be understood that certain embodiments can includemore elements than illustrated in a drawing and/or a subset of theelements illustrated in a drawing. Further, some embodiments canincorporate any suitable combination of features from two or moredrawings.

Acoustic wave filters can filter radio frequency (RF) signals in avariety of applications, such as in an RF front end of a mobile phone.An acoustic wave filter can be implemented with surface acoustic wave(SAW) devices. The SAW devices can be SAW resonators.

A composite substrate acoustic wave element that includes a lithiumtantalate (LiTaO₃) layer laminated on a silicon (Si) layer can suppressleakage in the substrate direction and achieve an improved qualityfactor (Q). In such an acoustic wave device, higher-order modes canoccur due to reflected waves generated at the interface between thelithium tantalate and silicon layers. The higher-order modes candeteriorate out-of-band attenuation characteristics. Such higher-ordermodes can be inhibited from occurring using spinel, which can have thechemical formula of MgAl₂O₄, instead of silicon (Si) in accordance withembodiments discussed herein.

Aspects of this disclosure relate to a multi-layer piezoelectricsubstrate that includes a lithium tantalate layer and a spinel layer,such as a polycrystalline spinel layer. A lithium tantalate layer on apolycrystalline spinel layer can inhibit higher-order modes fromoccurring in an acoustic wave device.

The propagation acoustic velocity of transverse-wave bulk waves inspinel (e.g., polycrystalline spinel and/or single crystalline spinel)is less than the propagation acoustic velocity of transverse-wave bulkwaves in silicon, whereas the propagation acoustic velocity oftransverse waves in spinel is greater than those of lithium tantalate.Accordingly, it is possible to lower the cutoff frequency of bulkemission in the substrate direction with a spinel layer withoutdeteriorating the shear horizontal (SH) mode. This can suppress theintensity of higher-order modes.

Aspects of this disclosure relate to structures to suppress the higherorder spurious modes for SAW filters on multi-layer substratestructures. For instance, SAW devices with a multi-layer piezoelectricthat includes a lithium based piezoelectric material, such as lithiumtantalate or lithium niobate, on silicon dioxide (SiO₂) on silicon canencounter higher order spurious modes. By replacing silicon with spinelin such multi-layer substrates, higher order spurious modes can besuppressed. The spinel layer can be a polycrystalline layer or a singlecrystalline layer. A number of SAW characteristics for spinel andsilicon are similar. One difference lies in the shear velocity, wherespinel is faster than silicon. Thus, the spurious modes can show up athigher frequencies on a spinel substrate compared to a siliconsubstrate. This characteristic can be used to suppress the higher orderspurious modes for SAW filters.

FIG. 1A illustrates a cross section of a baseline surface acoustic wavedevice 10. The baseline surface acoustic device 10 includes a siliconsubstrate 12, a lithium tantalate layer 14 having a thickness of H₁, andan interdigital transducer (IDT) electrode 16 on the lithium tantalatelayer 14 and having a thickness h. The illustrated IDT electrode 16 isan aluminum IDT electrode. The IDT electrode 16 has a pitch that setsthe wavelength λ or L of a surface acoustic wave generated by thesurface acoustic wave device 10. As illustrated, the wavelength λ of thesurface acoustic wave is 2 micrometers (um). The silicon substrate 12and the lithium tantalate layer 14 are bonded with each other and inphysical contact with each other in the surface acoustic wave device 10.

FIG. 1B illustrates a cross section of a surface acoustic wave device 20that includes a polycrystalline spinel substrate 22 according to anembodiment. The surface acoustic device 20 includes a polycrystallinespinel substrate 22, a lithium tantalate layer 14 having a thickness ofH₁, and an IDT electrode 16. The surface acoustic wave device 20 is likethe surface acoustic wave device 10 of FIG. 1A except that the surfaceacoustic wave device 20 includes the polycrystalline spinel substrate 22in place of the silicon substrate 12.

The polycrystalline spinel substrate 22 can suppress substrate leakageand contribute to the surface acoustic wave device achieving arelatively high Q and providing desirable out-of-band attenuationcharacteristics. The polycrystalline spinel substrate 22 can be bondedwith the lithium tantalate layer 14. The polycrystalline spinelsubstrate 22 can be in physical contact with the lithium tantalate layer14 as illustrated. The polycrystalline spinel layer 22 has a chemicalformula of MgAl₂O₄.

The thickness H₁ of the lithium tantalate layer 14 can be less than thewavelength λ of a surface acoustic wave generated by the surfaceacoustic wave device 20. This can contribute to the surface acousticwave device 20 achieving a desired electromechanical couplingcoefficient k². The lithium tantalate layer 14 is an example of apiezoelectric layer. A lithium niobate (LiNbO₃) piezoelectric layer canbe implemented in place of a lithium tantalate layer in any of theembodiments discussed herein. Any other suitable piezoelectric layer canbe implemented in place of the lithium tantalate layer 14.

The IDT electrode 16 is disposed on lithium tantalate layer 14. The IDTelectrode 16 has a pitch that sets the wavelength λ of a surfaceacoustic wave generated by the surface acoustic wave device 20. The IDTelectrode 16 can be an aluminum IDT electrode. IDT electrode materialcan include aluminum (Al), titanium (Ti), gold (Au), silver (Ag), copper(Cu), platinum (Pt), tungsten (W), molybdenum (Mo), ruthenium (Ru), orany suitable combination thereof. For instance, the IDT electrode 16 caninclude aluminum and molybdenum in certain applications. In someembodiments, the IDT electrode 16 can include multiple layers ofdifferent IDT electrode materials.

FIG. 2A is a graph comparing simulated frequency responses of thesurface acoustic wave devices 10 and 20 of FIGS. 1A and 1B,respectively. These simulations correspond to a wavelength λ of 2 um, anIDT electrode 16 having a thickness h of 150 nm, and a lithium tantalatelayer 14 having a thickness H1 of 1 um and being 42YX-lithium tantalate.The bulk wave cutoff frequency of polycrystalline spinel (e.g., about2.4 GHz) is lower than the bulk wave cutoff frequency for silicon (e.g.,about 2.9 GHz). Accordingly, a spurious mode that is present in thesurface acoustic wave device 10 can be suppressed in the surfaceacoustic wave device 20.

FIG. 2B is another graph comparing simulated frequency responses of thesurface acoustic wave devices 10 and 20 of FIGS. 1A and 1B,respectively. This graph indicates stronger suppression of a spuriousmode in the surface acoustic wave device 20 than in the surface acousticwave device 10.

FIG. 2C is a graph comparing admittance over frequency for a surfaceacoustic wave device of FIG. 1B and a similar device without a spinelsubstrate. The similar device is like the surface acoustic wave device20 except that the polycrystalline spinel substrate 22 is not included.The lithium tantalate layer of the similar device has a 42° cut angle.FIG. 2C indicates that the impedance ratio is improved by thepolycrystalline spinel layer 22 of FIG. 1B.

A temperature compensating layer with a positive temperature coefficientof frequency (TCF) can be added between a piezoelectric layer and thepolycrystalline spinel substrate of the surface acoustic wave device 20of FIG. 1B. This can bring the TCF of such a surface acoustic wavedevice closer to zero than the surface acoustic wave device 20 of FIG.1B. Accordingly, there can be less variation with temperature for thesurface acoustic wave device with the temperature compensating layer.This can be significant in certain applications.

The temperature compensating layer can be silicon dioxide (SiO₂) layer.The temperature compensating layer can be a layer of any other suitablematerial having a positive temperature coefficient of frequency. Forinstance, the temperature compensating layer can be a tellurium dioxide(TeO₂) layer or a silicon oxyfluoride (SiOF) layer in certainapplications. A temperature compensating layer can include any suitablecombination of, for example, SiO₂, TeO₂, and/or SiOF.

FIGS. 3A and 3B illustrate surface acoustic wave devices that include atemperature compensating layer in a multi-layer piezoelectric substrate.Any suitable principles and advantages of these surface acoustic wavedevices can be combined with each other and/or with any of the othersurface acoustic wave devices discussed herein.

FIG. 3A illustrates a cross section of a surface acoustic wave device 30that includes a polycrystalline spinel substrate 22 and a silicondioxide layer 32 according to an embodiment. The silicon dioxide layer32 can be any other suitable temperature compensating layer. The silicondioxide layer 32 can bring the TCF of the surface acoustic wave device30 closer to zero than the TCF of the surface acoustic wave device 20 ofFIG. 1B. The silicon dioxide layer 32 can improve the electromechanicalcoupling coefficient k² of the surface acoustic wave device 30 relativeto the surface acoustic wave device 20 of FIG. 1B.

FIG. 3B illustrates a cross section of a surface acoustic wave device 35that includes a polycrystalline spinel substrate 22, a silicon dioxidelayer 32, and a high impedance layer 36 according to an embodiment. Thesurface acoustic wave device 35 is like the surface acoustic wave device30 of FIG. 3A except that the high impedance layer 36 is additionallyimplemented. The high impedance layer 36 has a higher impedance than thepolycrystalline spinel substrate 22. The high impedance layer 36 canimprove the quality factor of the surface acoustic wave device 35relative to the surface acoustic wave device 30 of FIG. 3A. The highimpedance layer 36 can be a silicon nitride (SiN) layer, an aluminumnitride (AlN) layer, or the like.

Another substrate can be included below a polycrystalline spinelsubstrate on a side opposite to the piezoelectric layer of a surfaceacoustic wave device. In such surface acoustic wave devices, theadvantages of polycrystalline spinel can be realized and the othersubstrate can cause thermal dispersion and/or TCF to be improved.

FIGS. 4A to 4C illustrate surface acoustic wave devices that include amulti-layer piezoelectric substrate with a polycrystalline spinelsubstrate over another substrate. For example, in an embodiment, asurface acoustic wave device includes a support substrate (e.g. asilicon substrate), a polycrystalline layer over the support substrate,a piezoelectric layer (e.g., a lithium tantalate layer or a lithiumniobate layer) over the polycrystalline spinel layer, and aninterdigital transducer electrode over the piezoelectric layer. Anysuitable principles and advantages of these surface acoustic wavedevices can be combined with each other and/or with any of the othersurface acoustic wave devices discussed herein.

FIG. 4A illustrates a cross section of a surface acoustic wave device 40that includes a polycrystalline spinel substrate 22 over anothersubstrate 42 according to an embodiment. The surface acoustic wavedevice 40 is like the surface acoustic wave device 20 of FIG. 1B exceptthat the surface acoustic wave device 40 additionally includes the othersubstrate 42. As shown in FIG. 4A, polycrystalline spinel substrate 22is disposed between the other substrate 42 and a piezoelectric layer(e.g., the lithium tantalate layer 14). The other substrate 42 can serveas a support substrate. The other substrate 42 can be any suitablesubstrate that can improve thermal dispersion and/or TCF. For example,the other substrate 42 can be a silicon (Si) substrate, an aluminumnitride (AlN) substrate, a diamond substrate, a silicon nitride (SiN)substrate, a silicon carbide (SiC) substrate, or the like. The othersubstrate 42 can be a monocrystalline substrate.

FIG. 4B illustrates a cross section of a surface acoustic wave device 45that includes a polycrystalline spinel substrate 22 and a temperaturecompensating layer 32 over another substrate according to an embodiment.The surface acoustic wave device 45 is like the surface acoustic wavedevice 40 of FIG. 4A except that the surface acoustic wave device 45additionally includes the temperature compensating layer (e.g., asilicon dioxide layer 32) between the polycrystalline spinel substrate22 and the piezoelectric layer (e.g., the lithium tantalate layer 14).

FIG. 4C illustrates a cross section of a surface acoustic wave device 47that includes a polycrystalline spinel substrate 22, a temperaturecompensating layer, and a high impedance layer 36 over another substrate42 according to an embodiment. The surface acoustic wave device 47 islike the surface acoustic wave device 45 of FIG. 4B except that thesurface acoustic wave device 47 additionally includes a high impedancelayer 36 between the polycrystalline spinel substrate 22 and thetemperature compensating layer (e.g., a silicon dioxide layer 32).

Although embodiments discussed herein relate to a surface acoustic wavedevice that includes polycrystalline spinel layer that is bonded toanother layer of a multi-layer piezoelectric substrate, apolycrystalline spinel layer can be attached to another layer of amulti-layer piezoelectric substrate by an adhesive layer. FIGS. 5A to 5Cillustrate surface acoustic wave devices that include a polycrystallinespinel substrate attached to another substrate by an adhesive layer. Anysuitable principles and advantages of these surface acoustic wavedevices can be combined with each other and/or with any of the othersurface acoustic wave devices discussed herein.

FIG. 5A illustrates a cross section of a surface acoustic wave device 50that includes a polycrystalline spinel substrate 22 attached to anothersubstrate 42 by an adhesive layer 52. The surface acoustic wave device50 is like the surface acoustic wave device 40 of FIG. 4A except thatthe surface acoustic wave device 50 additionally includes the adhesivelayer 52.

FIG. 5B illustrates a cross section of a surface acoustic wave device 55that includes a polycrystalline spinel substrate 22 attached to anothersubstrate 42 by an adhesive layer 52. The surface acoustic wave device55 is like the surface acoustic wave device 50 of FIG. 5A except thatthe surface acoustic wave device 55 additionally includes a silicondioxide layer 32.

FIG. 5C illustrates a cross section of a surface acoustic wave device 57that includes a polycrystalline spinel substrate 22 attached to anothersubstrate 42 by an adhesive layer 52. The surface acoustic wave device57 is like the surface acoustic wave device 55 of FIG. 5B except thatthe surface acoustic wave device 57 additionally includes a highimpedance layer 36.

Although embodiments discussed herein relate to surface acoustic wavedevice that includes a polycrystalline spinel layer, a surface acousticwave device can include a single crystalline spinel layer instead. Forexample, FIGS. 6B and 6C illustrate surface acoustic wave devices thatinclude a single crystalline spinel substrate. In some embodiments, thepolycrystalline spinel layer may suppress higher-order modes more than asingle crystalline spinel layer. Any suitable principles and advantagesof the other surface acoustic wave devices discussed herein can beimplemented in association with the surface acoustic wave devices ofFIGS. 6B and/or 6C.

FIG. 6A illustrates a cross section of a baseline surface acoustic wavedevice 60. The baseline surface acoustic device 60 includes a siliconsubstrate 12, a silicon dioxide layer 32, a lithium tantalate layer 14having a thickness of H_LTO, and an aluminum IDT electrode 16 on thelithium tantalate layer 14. In the surface acoustic wave device 60, forexample, the silicon dioxide layer 32 has a thickness H_SiO₂ of 0.6 um,the aluminum IDT electrode 16 has a thickness of 160 nm and the IDTelectrode 16 has a pitch of 2 um that sets the wavelength λ or L of asurface acoustic wave generated by the surface acoustic wave device 60.

FIG. 6B illustrates a cross section of a surface acoustic wave device 65that includes a spinel substrate 67 according to an embodiment. Thespinel substrate 67 is a single crystalline spinel substrate. Asillustrated, the surface acoustic device 65 includes the spinelsubstrate 67, a lithium tantalate layer 14 having a thickness of H_LTO,a silicon dioxide layer 32 between the spinel substrate 67 and thelithium tantalate layer 14, and an IDT electrode 16. The surfaceacoustic wave device 65 is like the surface acoustic wave device 60 ofFIG. 6A except that the surface acoustic wave device 65 includes aspinel substrate 67 in place of the silicon substrate 12. The surfaceacoustic wave device 65 is like the surface acoustic wave device 30 ofFIG. 3A except that the surface acoustic wave device 65 includes thesingle crystalline spinel substrate 67 in place of the polycrystallinespinel substrate 22 and example dimensions for the surface acoustic wavedevice 65 are provided. In some embodiments, the lithium tantalate layer14 is 42YX-lithium tantalate. In the surface acoustic wave device 65,for example, the silicon dioxide layer 32 can have a thickness H_SiO₂ of0.6 um, the aluminum IDT electrode 16 can have a thickness of 160 nm andthe IDT electrode 16 can have a pitch of 2 um that sets the wavelength λor L of a surface acoustic wave generated by the surface acoustic wavedevice 65. Other layer thicknesses and/or pitch can be implemented. Thelithium tantalate layer 14 and the silicon dioxide layer 32 may havethicknesses H_LTO and H_SiO₂ that correspond to the pitch of the IDTelectrode 16 that sets the wavelength λ or L.

FIG. 6C illustrates a cross section of a surface acoustic wave device 70that includes a spinel substrate 67 according to an embodiment. Thespinel substrate 67 is a single crystalline spinel substrate. Asillustrated, the surface acoustic device 70 includes the spinelsubstrate 67, a lithium niobate layer 74 having a thickness of H_LN, asilicon dioxide layer 32 between the spinel substrate 67 and the lithiumniobate layer 74, and an IDT electrode 16. The surface acoustic wavedevice 70 is like the surface acoustic wave device 65 of FIG. 6B exceptthat the surface acoustic wave device 70 includes a differentpiezoelectric layer, namely the lithium niobate layer 74, in place ofthe lithium tantalate layer 14. In the surface acoustic wave device 70,as in other embodiments disclosed herein, various thicknesses and/orpitches can be implemented. For example, the lithium niobate layer 74and the silicon dioxide layer 32 may have thicknesses H_LN and H_SiO₂that correspond to a pitch of the IDT electrode 16 that sets thewavelength λ or L of a surface acoustic wave generated by the surfaceacoustic wave device 70.

As noted above, any suitable principles and advantages of the othersurface acoustic wave devices discussed herein can be implemented inassociation with the surface acoustic wave devices of FIGS. 6B and/or6C. In some embodiments, the spinel layer 67 of the surface acousticwave devices 65 and/or 70 may be disposed over another substrate similarto the other substrate 42 of the surface acoustic wave device 55 of FIG.5B. For example, the other substrate can be a silicon (Si) substrate, analuminum nitride (AlN) substrate, a diamond substrate, a silicon nitride(SiN) substrate, a silicon carbide (SiC) substrate, or the like. In someembodiments, a high impedance layer, similar to the high impedance layer36 of the surface acoustic wave device 57 of FIG. 5C, can be disposedbetween the silicon dioxide layer 32 and the spinel layer 67. Accordingto certain embodiments, the spinel layer 67 can be disposed between ahigh impedance layer and another substrate layer.

The surface acoustic wave devices 60 and 65 of FIGS. 6A and 6B,respectively, were simulated with various thicknesses H_LTO of a42YX-lithium tantalate layer. In these simulations, the materialproperties in Table 1 were used, the thickness H_SiO₂ of the silicondioxide layer 32 was 0.6 um, and pitch λ was 2 um.

TABLE 1 Shear V_(S2) Longitudinal V_(P) Material [m/s] [m/s] SiO₂ 21265501 Si 5086 13695 Spinel 5570 9782

FIG. 7A is a graph comparing simulated frequency responses of thesurface acoustic wave devices 60 and 65 of FIGS. 6A and 6B,respectively, for H_LTO=0.2 um. As shown in FIG. 7A, spurious modes areat higher frequencies for the surface acoustic wave device 65 than forthe surface acoustic wave device 60.

FIG. 7B is a graph comparing simulated frequency responses of thesurface acoustic wave devices 60 and 65 of FIGS. 6A and 6B,respectively, for H_LTO=1 um. FIG. 7B indicates that spurious modes areat higher frequencies for the surface acoustic wave device 65 than forthe surface acoustic wave device 60.

FIG. 7C is a graph comparing simulated frequency responses of thesurface acoustic wave devices 60 and 65 of FIGS. 6A and 6B,respectively, for H_LTO=2 um. Spurious modes are at higher frequenciesfor the surface acoustic wave device 65 than for the surface acousticwave device 60 in FIG. 7C.

FIG. 7D is a graph of various simulation results showing anelectromechanical coupling coefficient (K²) on a vertical axis and thethickness H_LTO of the lithium tantalate layer 14 on a horizontal axisfor the surface acoustic wave devices 65 of FIG. 6B for various silicondioxide layer thicknesses H_SiO₂/L=0 um, 0.1 um, 0.2 um, 0.3 um, 0.4 um,and 0.5 um. A line 76 at about K²=8.5% is shown for a surface acousticwave device with an IDT over a 42YX-lithium tantalate layer withoutother layers below the 42YX-lithium tantalate layer. For achievingdesirable K² values, the simulation results indicate that the layerthickness combinations in rows of Table 2 are desirable.

TABLE 2 Range for H_SiO₂ Corresponding Range for H_LTO   0λ < H_SiO₂ <0.05λ 0.17λ < H_LTO < 0.45λ 0.05λ < H_SiO₂ < 0.15λ 0.08λ < H_LTO < 0.45λ0.15λ < H_SiO₂ < 0.25λ 0.08λ < H_LTO < 0.42λ 0.25λ < H_SiO₂ < 0.35λ0.08λ < H_LTO < 0.41λ 0.35λ < H_SiO₂ < 0.45λ 0.08λ < H_LTO < 0.39λ 0.45λ< H_SiO2 < 0.55λ 0.08λ < H_LTO < 0.36λ

The acoustic wave devices disclosed herein can be implemented inacoustic wave filters arranged to filter radio frequency signals.Aspects of this disclosure relate to filtering a radio frequency signalwith an acoustic wave filter. A method can include providing a radiofrequency signal to an acoustic wave filter. The radio frequency signalcan provided via a radio frequency switch, for example. In someinstances, a power amplifier can provide the radio frequency signal tothe acoustic wave filter via the radio frequency switch. The methodincludes filtering the radio frequency signal with the acoustic wavefilter. The acoustic wave filter can include any suitable acoustic wavedevice disclosed herein. For example, the acoustic wave filter caninclude an acoustic wave device that includes a spinel layer, apiezoelectric layer, and an interdigital transducer electrode on thepiezoelectric layer, in which the piezoelectric layer is disposedbetween the interdigital transducer electrode and the spinel layer. Thepiezoelectric layer can have a thickness than is less than λ, in whichthe acoustic wave device is configured to generate an acoustic wavehaving a wavelength of λ. The spinel layer can be a polycrystallinespinel layer. The acoustic wave device can include a temperaturecompensating layer, such as a silicon dioxide layer, between thepiezoelectric layer and the spinel layer. In the method, the radiofrequency signal can be filtered while higher-order modes aresuppressed.

The acoustic wave devices discussed herein can be implemented in avariety of packaged modules. A packaged module configured to process aradio frequency signal can be referred to as a radio frequency module.Some example packaged modules will now be discussed in which anysuitable principles and advantages of the acoustic wave devicesdiscussed herein can be implemented. FIGS. 8A to 8F are schematic blockdiagrams of illustrative packaged modules according to certainembodiments. Any suitable combination of features of these embodimentscan be combined with each other. FIGS. 8A to 8C illustrate modules thatinclude filters that includes an acoustic device in accordance with theprinciples and advantages disclosed herein. FIGS. 8D to 8F illustratemodules that include duplexers that includes an acoustic device inaccordance with the principles and advantages disclosed herein. AlthoughFIGS. 8D to 8F illustrate duplexers, any suitable principles andadvantages disclosed herein can be implemented in other multiplexers(e.g., quadplexers, hexaplexers, octoplexers, etc.) and/or inswitch-plexers.

FIG. 8A is a schematic block diagram of a module 80 that includesfilters 82 and a switch 83. The module 80 can include a package thatencloses the illustrated elements. The filters 82 and the switch 83 canbe disposed on a common packaging substrate. The packaging substrate canbe a laminate substrate, for example. The switch 83 can be a multi-throwradio frequency switch. The switch 83 can electrically couple a selectedfilter of the filters 82 to a common node. The common node can be anantenna node, for example. As another example, the common node can becoupled to an output of a power amplifier. The filters 82 can includeany suitable number of acoustic wave filters. One or more of theacoustic wave filters of the filters 82 can be implemented in accordancewith any suitable principles and advantages of the acoustic wave devicesdiscussed herein.

FIG. 8B is a schematic block diagram of a module 84 that includes apower amplifier 85, a switch 86, and filters 82 in accordance with oneor more embodiments. The module 84 can include a package that enclosesthe illustrated elements. The power amplifier 85, the switch 86, and thefilters 82 can be disposed on a common packaging substrate. Thepackaging substrate can be a laminate substrate, for example. The switch86 can be a multi-throw radio frequency switch. The switch 86 canelectrically couple an output of the power amplifier 85 to a selectedfilter of the filters 82. The filters 82 can include any suitable numberof acoustic wave filters. One or more of the acoustic wave filters ofthe filters 82 can be implemented in accordance with any suitableprinciples and advantages of the acoustic wave devices discussed herein.

FIG. 8C is a schematic block diagram of a module 88 that includes poweramplifiers 85A and 85B, switches 86A and 86B, and filters 82A and 82B,and switch 89. The module 88 can include a package that encloses theillustrated elements. The illustrated elements can be disposed on acommon packaging substrate. The packaging substrate can be a laminatesubstrate, for example. The module 88 is like the module 84 of FIG. 8B,except that the module 88 includes an additional power amplifier 85B, anadditional switch 86B, additional filters 82B, and an antenna switch 89.The antenna switch 89 can selectively couple a signal from the filters82A or the filters 82B to an antenna node. The different signal pathscan be associated with different frequency bands and/or different modesof operation (e.g., different power modes, different signaling modes,etc.).

FIG. 8D is a schematic block diagram of a module 110 that includesduplexers 112 and a switch 83. The module 110 can include a package thatencloses the illustrated elements. The duplexers 112 and the switch 83can be disposed on a common packaging substrate. The packaging substratecan be a laminate substrate, for example. The duplexers 112 can includetwo or more filters coupled to a common node. The switch 83 canelectrically couple a selected duplexer of the duplexers 112 to a commonnode. The common node can be an antenna node, for example. As anotherexample, the common node can be coupled to an output of a poweramplifier.

One or more duplexers of the duplexers 112 can be implemented by anyother suitable multiplexer that includes a plurality of filters coupledto each other at a common node. In some embodiments, such a multiplexercan be a quadplexer. The multiplexer can be a pentaplexer. Themultiplexer can be a hexaplexer. The multiplexer can be a heptaplexer.The multiplexer can be an octoplexer.

FIG. 8E is a schematic block diagram of a module 114 that includes apower amplifier 85, a switch 86, and duplexers 112 in accordance withone or more embodiments. The module 114 can include a package thatencloses the illustrated elements. The power amplifier 85, the switch86, and the duplexers 112 can be disposed on a common packagingsubstrate. The packaging substrate can be a laminate substrate, forexample. The switch 86 can be a multi-throw radio frequency switch. Theswitch 86 can electrically couple an output of the power amplifier 85 toa selected duplexer of the duplexers 112. The duplexers 112 can eachinclude two acoustic wave filters coupled to a common node. The twoacoustic wave filters can be a transmit filter and a receive filter.

FIG. 8F is a schematic block diagram of a module 118 that includes poweramplifiers 85A and 85B, switches 86A and 86B, and duplexers 112A and112B, and switch 89. The module 118 can include a package that enclosesthe illustrated elements. The illustrated elements can be disposed on acommon packaging substrate. The packaging substrate can be a laminatesubstrate, for example. The module 118 is like the module 114 of FIG.8E, except that the module 118 includes an additional power amplifier85B, an additional switch 86B, additional duplexers 112B, and an antennaswitch 89. The antenna switch 89 can selectively couple a signal fromthe duplexers 112A or the duplexers 112B to an antenna node. Thedifferent signal paths can be associated with different frequency bandsand/or different modes of operation (e.g., different power modes,different signaling modes, etc.).

FIG. 9A is a schematic diagram of a wireless communication device 90that includes a filter 93 in a radio frequency front end 92 according toan embodiment. The filter 93 can include an acoustic wave device inaccordance with any suitable principles and advantages discussed herein.The wireless communication device 90 can be any suitable wirelesscommunication device. For instance, a wireless communication device 90can be a mobile phone, such as a smart phone. In some other instances,the wireless communication device 90 can be an Internet of thingsdevice, a laptop or notebook computer, or a tablet computer. Asillustrated, the wireless communication device 90 includes an antenna91, an RF front end 92, a transceiver 94, a processor 95, a memory 96,and a user interface 97. The antenna 91 can transmit RF signals providedby the RF front end 92. Such RF signals can include carrier aggregationsignals. The antenna 91 can provide received RF signals to the RF frontend 92 for processing. Such RF signals can include carrier aggregationsignals.

The RF front end 92 can include one or more power amplifiers, one ormore low noise amplifiers, RF switches, receive filters, transmitfilters, duplex filters, multiplexers, frequency multiplexing circuits,or any combination thereof. The RF front end 92 can transmit and receiveRF signals associated with any suitable communication standards. Thefilter 93 can include a surface acoustic wave device that includes anysuitable combination of features discussed with reference to anyembodiments discussed above.

The transceiver 94 can provide RF signals to the RF front end 92 foramplification and/or other processing. The transceiver 94 can alsoprocess an RF signal provided by a low noise amplifier of the RF frontend 92. The transceiver 94 is in communication with the processor 95.The processor 95 can be a baseband processor. The processor 95 canprovide any suitable base band processing functions for the wirelesscommunication device 90. The memory 96 can be accessed by the processor95. The memory 96 can store any suitable data for the wirelesscommunication device 90. The user interface 97 can be any suitable userinterface, such as a display with touch screen capabilities.

FIG. 9B is a schematic diagram of a wireless communication device 100that includes a filter 93 in a radio frequency front end 92 and a secondfilter 103 in a diversity receive module 102. The wireless communicationdevice 100 is like the wireless communication device 90 of FIG. 9A,except that the wireless communication device 100 also includesdiversity receive features. As illustrated in FIG. 9B, the wirelesscommunication device 100 includes a diversity antenna 101, a diversitymodule 102 configured to process signals received by the diversityantenna 101 and including the second filter 103, and a transceiver 104in communication with both the radio frequency front end 92 and thediversity receive module 102. The second filter 103 can include asurface acoustic wave device that includes any suitable combination offeatures discussed with reference to any embodiments discussed above.

Although embodiments are discussed with reference to surface acousticwave devices, any suitable principles and advantages discussed hereincan be applied to boundary acoustic wave devices, Lamb wave resonators,and/or other suitable acoustic resonators.

Any of the embodiments described above can be implemented in associationwith mobile devices such as cellular handsets. The principles andadvantages of the embodiments can be used for any systems or apparatus,such as any uplink cellular device, that could benefit from any of theembodiments described herein. The teachings herein are applicable to avariety of systems. Although this disclosure includes some exampleembodiments, the teachings described herein can be applied to a varietyof structures. Any of the principles and advantages discussed herein canbe implemented in association with RF circuits configured to processsignals having a frequency in a range from about 30 kHz to 300 GHz, suchas a frequency in a range from about 450 MHz to 6 GHz.

Aspects of this disclosure can be implemented in various electronicdevices. Examples of the electronic devices can include, but are notlimited to, consumer electronic products, parts of the consumerelectronic products such as die and/or acoustic wave filter assembliesand/or packaged radio frequency modules, uplink wireless communicationdevices, wireless communication infrastructure, electronic testequipment, etc. Examples of the electronic devices can include, but arenot limited to, a mobile phone such as a smart phone, a wearablecomputing device such as a smart watch or an ear piece, a telephone, atelevision, a computer monitor, a computer, a modem, a hand-heldcomputer, a laptop computer, a tablet computer, a personal digitalassistant (PDA), a microwave, a refrigerator, an automobile, a stereosystem, a DVD player, a CD player, a digital music player such as an MP3player, a radio, a camcorder, a camera, a digital camera, a portablememory chip, a washer, a dryer, a washer/dryer, a copier, a facsimilemachine, a scanner, a multi-functional peripheral device, a wrist watch,a clock, etc. Further, the electronic devices can include unfinishedproducts.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,”“include,” “including” and the like are to be construed in an inclusivesense, as opposed to an exclusive or exhaustive sense; that is to say,in the sense of “including, but not limited to.” The word “coupled”, asgenerally used herein, refers to two or more elements that may be eitherdirectly connected, or connected by way of one or more intermediateelements. Likewise, the word “connected”, as generally used herein,refers to two or more elements that may be either directly connected, orconnected by way of one or more intermediate elements. Additionally, thewords “herein,” “above,” “below,” and words of similar import, when usedin this application, shall refer to this application as a whole and notto any particular portions of this application. Where the contextpermits, words in the above Detailed Description using the singular orplural number may also include the plural or singular numberrespectively. The word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel apparatus, methods, andsystems described herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofthe methods and systems described herein may be made without departingfrom the spirit of the disclosure. For example, while blocks arepresented in a given arrangement, alternative embodiments may performsimilar functionalities with different components and/or circuittopologies, and some blocks may be deleted, moved, added, subdivided,combined, and/or modified. Each of these blocks may be implemented in avariety of different ways. Any suitable combination of the elements andacts of the various embodiments described above can be combined toprovide further embodiments. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of the disclosure.

What is claimed is:
 1. An acoustic wave device comprising: apolycrystalline spinel layer; a piezoelectric layer having a thicknessthan is less than λ, the acoustic wave device being configured togenerate an acoustic wave having a wavelength of λ; and an interdigitaltransducer electrode on the piezoelectric layer, the piezoelectric layerbeing disposed between the interdigital transducer electrode and thepolycrystalline spinel layer.
 2. The acoustic wave device of claim 1further comprising a temperature compensating layer disposed between thepolycrystalline spinel layer and the piezoelectric layer.
 3. Theacoustic wave device of claim 2 wherein the temperature compensatinglayer includes silicon dioxide.
 4. The acoustic wave device of claim 2further comprising a high impedance layer disposed between thetemperature compensating layer and the polycrystalline spinel layer, thehigh impedance layer having a higher acoustic impedance than thepolycrystalline spinel layer.
 5. The acoustic wave device of claim 2wherein the temperature compensating layer is in physical contact withthe piezoelectric layer.
 6. The acoustic wave device of claim 5 whereinthe temperature compensating layer is in physical contact with thepolycrystalline spinel layer opposite to the piezoelectric layer.
 7. Theacoustic wave device of claim 1 wherein the polycrystalline spinel layeris in physical contact with the piezoelectric layer.
 8. The acousticwave device of claim 1 wherein the piezoelectric layer is a lithiumtantalate layer.
 9. The acoustic wave device of claim 1 wherein thepiezoelectric layer is a lithium niobate layer.
 10. The acoustic wavedevice of claim 1 further comprising a substrate layer, thepolycrystalline spinel layer being disposed between the substrate layerand the piezoelectric layer.
 11. The acoustic wave device of claim 10wherein the substrate layer is a silicon layer.
 12. The acoustic wavedevice of claim 10 wherein the substrate layer is in physical contactwith the polycrystalline spinel layer.
 13. The acoustic wave device ofclaim 10 further comprising an adhesive layer attaching the substratelayer and the polycrystalline spinel layer.
 14. The acoustic wave deviceof claim 1 wherein the acoustic wave device is a surface acoustic wavedevice, and the acoustic wave is a surface acoustic wave.
 15. Theacoustic wave device of claim 1 wherein the polycrystalline spinel layerincludes spinel having a chemical formula of MgAl₂O₄.
 16. A radiofrequency module comprising: an acoustic wave filter configured tofilter a radio frequency signal, the acoustic wave filter including anacoustic wave device configured to generate an acoustic wave having awave length of λ, the acoustic wave device including a piezoelectriclayer having a thickness of less than λ, an interdigital transducerelectrode on the piezoelectric layer, and a polycrystalline spinel layerarranged such that the piezoelectric layer is disposed between thepolycrystalline spinel layer and the interdigital transducer electrode;and a package enclosing the acoustic wave filter.
 17. The radiofrequency module of claim 16 further comprising a second acoustic wavefilter, the acoustic wave filter and the second acoustic wave filterbeing included in a duplexer.
 18. The radio frequency module of claim 16further comprising a power amplifier enclosed within the package, thepower amplifier configured to provide the radio frequency signal. 19.The radio frequency module of claim 16 wherein the acoustic wave filteris a surface acoustic wave filter and the piezoelectric layer is alithium-based piezoelectric layer.
 20. A wireless communication devicecomprising: an antenna; and an acoustic wave filter in communicationwith the antenna, the acoustic wave filter being configured to filter aradio frequency signal, the acoustic wave filter including an acousticwave device configured to generate an acoustic wave having a wave lengthof λ, the acoustic wave device including a piezoelectric layer having athickness of less than λ, an interdigital transducer electrode on thepiezoelectric layer, and a polycrystalline spinel layer arranged suchthat the piezoelectric layer is disposed between the polycrystallinespinel layer and the interdigital transducer electrode.
 21. The wirelesscommunication device of claim 20 wherein the acoustic wave filter is asurface acoustic wave filter.